Method of pelletization of polymer

ABSTRACT

A method of preparing cellular pellets from a prepolymer comprising a blowing agent, the method comprising the steps of: 
     a) extruding the prepolymer through a die, the die maintained at conditions such that the blowing agent remains in the condensed phase in the prepolymer prior to emerging from the die, and 
     b) upon emergence of the prepolymer through the die, substantially simultaneously cooling the prepolymer by contacting the prepolymer with a cooling agent and cutting the prepolymer; the conditions outside the die being maintained such that the blowing agent vaporizes in the prepolymer to form pores.

BACKGROUND OF THE INVENTION

In solid state polymerization, or “SSP”, a high molecular weight polymeris produced by first preparing a relatively low molecular weightcrystallized prepolymer followed by its conversion to a higher molecularweight material in the solid state. A solid phase polymerization processis made possible by the ability of both the starting molecular weightmaterial and the product high polymer to sustain temperatures above theglass transition temperature without fusion of the polymer. Thetemperature at which the process is conducted must be sufficient toeffect chain growth and the attendant increase in M_(w).

Solid state polymerization (SSP) of polycarbonates is disclosed, forexample, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,214,073.Typically, SSP involves three steps: a first step of forming aprepolymer, typically by melt polymerization (i.e. transesterification)of a dihydroxyaromatic compound such as bisphenol A with a diarylcarbonate, such as diphenyl carbonate; a second step of crystallizingthe prepolymer; and a third step of building the molecular weight of thecrystallized prepolymer by heating to a temperature between its glasstransition temperature and its melting temperature. The second orcrystallization step as disclosed in these patents is by solventtreatment or heat treatment.

There exists a need for an improved method for crystallizing thepolycarbonate prepolymer.

There exists a further need for a prepolymer in a form which may bereadily processed to produce polycarbonate having improved properties.

There exists a further need for a process in which the in which the rateof solid state polymerization may be effectively increased.

There further exists a need for a SSP process which may be conducted atshorter processing times.

There further exists a need for a SSP process in which the process maybe conducted in fewer stages.

BRIEF SUMMARY OF THE INVENTION

In a first aspect, the invention relates to a method of crystallizing anaromatic polycarbonate prepolymer having a weight average molecularweight of from about 1,000 to about 20,000 and having from about 5 toabout 95 mole % aryl carbonate terminal end groups, based on total endgroups, the method comprising:

a) contacting the prepolymer with a crystallization agent comprising analcohol or alcohols under conditions effective to crystallize theprepolymer.

Optionally, the crystallization agent may comprise an additive. Thecrystallization agent preferably comprises at least 95% by weight of thealcohol or alcohols.

In one embodiment of this aspect of the invention, the invention relatesto a method of crystallizing an aromatic polycarbonate prepolymer havinga weight average molecular weight of from about 1,000 to about 20,000and having from about 5 to about 95 mole % aryl carbonate terminal endgroups, based on total end groups, the method comprising:

a) contacting the prepolymer with an agent comprising a secondaryalcohol, wherein the secondary alcohol is applied to the prepolymer inthe vapor phase.

In a further embodiment, this aspect of the invention relates to amethod a method of increasing the rate of solid state polymerizationduring preparation of an aromatic polycarbonate from a prepolymer, themethod comprising:

a) contacting the prepolymer with a crystallizing agent comprising analcohol and an additive, the additive comprising a substance effectiveto increase the rate of solid state polymerization.

In a further embodiment, this aspect of the invention relates to amethod of controlling the thermal crystallization of a prepolymer havinga weight average molecular weight of from about 1,000 to about 20,000and having from about 5 to about 95 mole % aryl carbonate terminal endgroups, based on total end groups, the method comprising the step of:

a) applying a crystallizing agent comprising an alcohol or alcohols tothe prepolymer.

Optionally, the crystallization agent may comprise an additive. Thecrystallization agent preferably comprises at least about 95% by weightof the alcohol or alcohols.

In a second aspect, the invention relates to a method of preparingcellular pellets from a prepolymer comprising a blowing agent, themethod comprising the steps of:

a) extruding the prepolymer through a die, the die maintained atconditions such that the blowing agent remains in the condensed phase inthe prepolymer prior to emerging from the die, and

b) upon emergence of the prepolymer through the die, substantiallysimultaneously cooling the prepolymer by contacting the prepolymer witha cooling agent and cutting the prepolymer; the conditions outside thedie being maintained such that the blowing agent vaporizes in theprepolymer to form pores.

In one embodiment, the second aspect of the invention relates to amethod of preparing cellular pellets comprising an aromaticpolycarbonate prepolymer having a weight average molecular weight offrom about 1,000 to about 20,000 and having from about 5 to about 95mole % aryl carbonate terminal end groups, based on total end groups,the aromatic polycarbonate prepolymer further comprising a blowingagent, the method comprising:

a) extruding the aromatic polycarbonate prepolymer through a die, thedie maintained at conditions such that the blowing agent remains in thecondensed phase prior to emerging from the die, and

b) upon emergence of the aromatic polycarbonate prepolymer through thedie, substantially simultaneously cooling the aromatic polycarbonateprepolymer by contacting the aromatic polycarbonate prepolymer with acooling agent and cutting the aromatic polycarbonate prepolymer; theconditions outside the die being maintained such that the blowing agentvaporizes in the aromatic polycarbonate prepolymer to form pores.

In a further embodiment of this second aspect of the invention, thecellular pellets are crystallized by effecting contact with an agentcomprising an alcohol and further subjected to a process of solid statepolymerization.

Additional advantages of the invention will be set forth in thedescription which follows. The advantages of the invention will berealized and attained by means of the elements and combinations setforth in the appended claims. It is to be understood that both theforegoing general description and the following detailed description areexemplary only, and not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of plot of the saturation vapor pressure of purephenol and pure diphenyl carbonate versus temperature. Phenol anddiphenyl carbonates are by-products of aromatic polycarbonate synthesis.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be understood more readily by reference to thefollowing detailed description of the preferred embodiments of theinvention and the examples included therein.

Before the present method and apparatus are disclosed and described, itis to be understood that this invention is not limited to specificsystemic methods or to particular formulations, as such may, of course,vary. It is also to be understood that the terminology used herein isfor the purpose of describing particular embodiments only and is notintended to be limiting.

In this specification and in the claims which follow, reference will bemade to a number of terms which shall be defined to have the followingmeaning.

The singular forms “a”, “an” and “the” include plural referents unlessthe context clearly dictates otherwise.

“Optional” or “optionally” mean that the subsequently described event orcircumstance may or may not occur, and that the description includesinstances where said event or circumstance occurs and instances where itdoes not.

“Nonsolvent” is herein defined as a substance having a prepolymersolubility of less than 10%.

“Solvent” is herein defined as substance that penetrates the prepolymer.

“Cellular pellet” is herein defined as a pellet containing internalvoids.

“Essentially pure” is herein defined as 99% by weight or greater of thereferenced material.

“Agglomerated pellet” is herein defined as a pellet formed from powderparticulate which has been densified.

In a first aspect, the present invention concerns a method ofcrystallizing a polycarbonate prepolymer by effecting contact betweenthe prepolymer and an agent comprising an alcohol. Optionally, thecrystallization agent may further comprise an additive. Thepolycarbonate prepolymer may be further polymerized by SSP.

In a second aspect, the invention concerns the preparation of cellularpellets from a prepolymer by a pelletization process. In one embodiment,the prepolymer may be polycarbonate. The cellular pellets may optionallybe treated by effecting contact of the cellular pellets with an agentcomprising an alcohol. The prepolymer may be further polymerized by SSP.

More particularly, the second aspect of the invention includes a methodin which a polymer feedstock comprising a blowing agent is processedunder conditions such that a cellular pellet is formed. In oneembodiment, the blowing agent is present in the polymer feedstock as abyproduct of the preparation of the polymer feedstock. In anotherembodiment, the blowing agent may be introduced into the feedstock.

Although the method of making the cellular pellets is not limited topolycarbonate prepolymer, the method is suitable for preparing cellularpellets of polycarbonate prepolymer, which may be crystallized andsubjected to further molecular weight increase by SSP. Optionally, thecrystallization of the cellular pellets may be effected by treatmentwith the crystallization agent comprising an alcohol as described in thefirst aspect of the invention.

I. Polycarbonate Prepolymer Crystallization

As mentioned, in a first aspect, the invention concerns a method ofcrystallization of a polycarbonate prepolymer comprising effectingcontact of the polycarbonate prepolymer with an agent comprising analcohol. This agent is herein referred to as a “crystallizing agent” ora “crystallization agent”. The agent may optionally comprise an additiveor additives. The polycarbonate prepolymer that is treated with thecrystallization agent may be in any form suitable for processing,including, but not limited to, a powder, a pellet, an agglomeratedpellet, or a cellular pellet.

In one embodiment of the present invention there is provided a method ofcrystallizing an aromatic polycarbonate prepolymer having a weightaverage molecular weight in the range of about 1,000 to about 20,000,and having in the range of about 5 to about 95 mole % aryl carbonateterminal end groups based on total end groups. The method generallyincludes the step of effecting contact of the prepolymer with acrystallizing agent comprising an alcohol to form a crystallizedprepolymer. Optionally, the prepolymer may first be obtained bycontacting a dihydroxy diaryl compound with a diaryl carbonate to form apolycarbonate prepolymer having a weight average molecular weight in therange of about 1,000 to about 20,000 and having in the range of about 5to about 95 mole % aryl carbonate terminal end groups based on total endgroups.

The polycarbonate prepolymer typically comprises structural units of theformula III

wherein at least about 60% of the total number of R groups are aromaticorganic radicals and the balance thereof are aliphatic, alicyclic oraromatic radicals. Preferably, each R is an aromatic organic radical andmore preferably a radical of the formula

—A₁—Y—A₂—  IV

wherein each A₁ and A₂ is a monocyclic divalent aryl radical and Y is abridging radical in which one or two carbonate atoms separate A₁ and A₂.Such radicals may be derived from dihydroxyaromatic compounds of theformulas OH—R—OH and OH—A₁—Y—A₂—OH, or their corresponding derivatives.A₁ and A₂ include but are not limited to unsubstituted phenylene,preferably p-phenylene or substituted derivatives thereof. The bridgingradical Y is most often a hydrocarbon group and preferably a saturatedgroup, such as methylene, cyclohexylidene or isopropylidene.Isopropylidene is the more preferred. Thus, the more preferredpolycarbonates are those comprising residues of2,2-bis(4-hydroxyphenyl)propane, also known as “bisphenol A”. In oneembodiment, the prepolymer is a homopolymer of bisphenol A. It ispreferable that the aromatic polycarbonate prepolymer have a T_(g) offrom about 100° C. to about 150° C., more preferably about 110° C.

A prepolymer of a (co)polyestercarbonates may also be prepared by themethod of this invention. The polyestercarbonate may comprise residuesof aliphatic or aromatic diacids. The corresponding derivatives ofaliphatic or aromatic diacids, such as the corresponding dichlorides,may also be utilized in the polymerization.

Polyfunctional compounds may also be introduced into the reaction toproduce, for example, branched polycarbonate prepolymer.

Suitable bisphenols or diphenols which may be used in the preparation ofthe prepolymer include, but are not limited to,

resourcinol,

4-bromoresourcinol,

hydroquinone,

4,4′-dihydroxybiphenyl,

1,6-dihydroxynapthalene,

bis(4-hydroxyphenyl)methane,

bis(4-hydroxyphenyl)diphenylmethane,

bis(4-hydroxyphenyl)-1-napthylmethane,

1,1-bis(4-hydroxyphenyl)ethane,

1,2-bis(4-hydroxyphenyl)ethane,

1,1-bis(4-hydroxyphenyl)phenylethane,

2,2-bis(4-hydroxyphenyl)propane (“bisphenol A”),

2-(4-hydroxyphenyl)-2-)3-hydroxyphenyl)propane,

2,2-bis(4-hydroxyphenyl)butane,

1,1-bis(4-hydroxyphenyl)isobutane,

1,1-bis(4-hydroxyphenyl)cyclohexane,

trans-2,3-bis(4-hydroxyphenyl)-2-butene,

2,2-bis(4-hydroxyphenyl)adamantane,

α,α′-bis(4-hydroxyphenyl)toluene,

bis(4-hydroxyphenyl)acetonitrile,

2,2-bis(3-methyl-4-hydroxyphenyl)propane,

2,2-bis(3-ethyl-4-hydroxyphenyl)propane,

2,2-bis(3-n-propyl-4-hydroxyphenyl)propane,

2,2-bis(3-sec-butyl-4-hydroxyphenyl)propane,

2,2-bis(3-t-butyl -4-hydroxyphenyl)propane,

2,2-bis(3-cyclohexyl-4-hydroxyphenyl)propane,

2,2-bis(3-allyl-4-hydroxyphenyl)propane,

2,2-bis(3-methoxy-4-hydroxyphenyl)propane,

2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane,

2,2-bis(2,3,5,6-tetramethyl-4-hydroxyphenyl)propane,

2,2-bis(3,5-dichloro-4-hydroxyphenyl)propane,

2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane,

2,2-bis(2,6-dibromo-3,5-dimethyl-4-hydroxyphenyl)propane,

α,α-bis(4-hydroxyphenyl)toluene,

α,α,α′,α′-tetramethyl-α,α′-bis(4-hydroxyphenyl)p-xylene,

2,2-bis(4-hydroxyphenyl)hexafluoropropane,

1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene,

1,1-dibromo-2,2-bis(4-hydroxyphenyl)ethylene,

1,1-dichloro-2,2-bis(5-phenoxy-4-hydroxyphenyl)ethylene,

4,4′-dihydroxybenzophenone,

3,3-bis(4-hydroxyphenyl)-2-butanone,

1,6-bis(4-hydroxyphenyl)- 1,6-hexanedione,

ethylene glycol bis(4-hydroxyphenyl)ether,

bis(4-hydroxyphenyl)ether,

bis(4-hydroxyphenyl)sulfide,

bis(4-hydroxyphenyl)sulfoxide,

bis(4-hydroxyphenyl)sulfone,

bis(3,5-dimethyl-4-hydroxyphenyl)sulfone,

9,9-bis(4-hydroxyphenyl)fluorene,

2,7-dihydroxypyrene,

6,6′-dihydroxy-3,3,3′,3′-tetramethylspiro(bis)indane (“spirobiindaneisphenol”),

3,3-bis(4-hydroxyphenyl)phthalide,

2,6-dihydroxydibenzo-p-dioxin,

2,6-dihydroxythianthrene,

2,7-dihydroxyphenoxathiin,

2,7-dihydroxy-9,10-methylphenazine,

3,6-dihydroxydibenzofuran,

3,6-dihydroxydibenzothiophene,

2,7-dihydroxycarbazole,

4,4-bis(4-hydroxyphenyl)heptane,

2,2-bis(4-hydroxyphenyl)hexane,

and other halogenated or alkylated derivatives. It is also possible touse mixtures of mono- and/or bischloroformates of the desired bisphenolor mono- and/or bischloroformate oligomeric carbonate mixtures of thedesired bisphenol. 2,2- bis(4-hydroxyphenyl)propane (or bisphenol A) isthe preferable bisphenol.

Suitable polyfunctional compounds used in the polymerization of branchedpolycarbonate include, but are not limited to,

1,1,1-tris(4-hydroxyphenyl)ethane,

4-[4-[1,1-bis(4-hydroxyphenyl)-ethyl]-dimethylbennzyl],

trimellitic anhydride,

trimellitic acid, or their acid chloride derivatives.

Suitable dicarboxylic acids or dicarboxylic dichlorides which may beused with bisphenols in the polymerization of polyester carbonatesinclude, but are not limited to,

1,10-decane dicarboxylic acid,

1,12- dodecane dicarboxylic acid,

terephthalic acid,

isophthalic acid,

terephthaloyl dichloride, and isophthaloyl dichloride.

It should be understood that any suitable method of making thepolycarbonate prepolymer, including catalytic and noncatalytic methodsmay be utilized in the present invention. Such methods include, but arenot limited to the interfacial process and the melt process. It ispreferable to conduct the preliminary polymerization in a molten stateas it typically yields a mixture of capped and uncapped oligomers, andthe melt process does not require the introduction of chlorinatedsolvents.

The crystallization agent comprises a primary alcohol, more preferably atertiary alcohol, even more preferably a secondary alcohol. Mixtures ofprimary, secondary and tertiary alcohols and any combination thereof mayalso be used in the crystallization agent. In one embodiment of theinvention, the crystallization agent is essentially pure alcohol.Alcohol is a nonsolvent for the polycarbonate prepolymer. In particular,the alcohols of the crystallization agent have a prepolymer solubilityof less than about 10%, preferably less than about 5%.

If primary alcohols are present in the crystallization agent the primaryalcohol or alcohols preferably a boiling point of less than about 250°C., more preferably less than about 200° C., even more preferably lessthan about 180° C. Suitable primary alcohols include C₁-C₁₀ alcoholsincluding, but are not limited to, methanol, ethanol, propanol, butanol,pentanol, hexanol, isobutanol, neopentyl alcohol and mixtures thereof.Methanol, butanol, pentanol and mixtures thereof are the more preferred.In one embodiment of the invention, branched primary alcohols areutilized.

If secondary alcohols are present in the crystallization agent, thesecondary alcohol or alcohols preferably having a boiling point of lessthan about 250° C., more preferably less than about 200° C., even morepreferably less than about 180° C. Suitable secondary alcohols includeC₁-C₁₀ alcohols, including but not limited to, 2-propanol (isopropanol),3-pentanol, sec-butanol, 2-octanol, 2-decanol and mixtures thereof.Isopropanol is preferred.

If tertiary alcohols are present in the crystallization agent, thetertiary alcohol or alcohols preferably having a boiling point of lessthan about 250° C., preferably less than about 200° C., even morepreferably less than about 180° C. Suitable tertiary alcohols includeC₁-C₁₀ alcohols, including but not limited to, t-butanol.

In addition to the primary, secondary and tertiary alcohols, diols andtriols may also optionally be used in the crystallization agent.Suitable diols include, but are not limited to any aliphatic orcycloaliphatic diol having from about 2 to a about 10 carbon atoms andmixtures thereof. Preferred diols include ethylene diol,1,3-trimethylene diol, propylene diol, tripropylene diol,1,4-butanediol, diethylene diol, neopentyl diol and mixtures thereof.

The crystallization agent may optionally comprise one or more polyolcomponents. A polyol is herein defined as a polyhydric alcoholcontaining three or more hydroxyl groups; while a diol is herein definedas a dihydric alcohol. Representative polyol components that may be usedin the crystallization agent include, but are not limited to, glycerol,trimethylolpropanate, pentaerythritol, 1,2,6-hexanetriol,dipentaerythritol and mixtures thereof.

The crystallization agent preferably comprises at least about 95% byweight of alcohol, more preferably at least about 99% by weight of thealcohol. The crystallization agent may be an essentially pure primaryalcohol, an essentially pure tertiary alcohol, or an essentially puresecondary alcohol. In one embodiment of the invention thecrystallization agent is essentially pure secondary alcohol applied inthe vapor phase.

The crystallization agent which contacts the prepolymer may be in theliquid phase or the vapor phase. By “vapor phase” it is to be understoodthat the crystallization agent is in a single phase, and is in the formof a vapor. Optionally, contact of the crystallization agent comprisingthe alcohol may be effected under conditions such that there is liquidin the system as well.

Contact of the crystallizing agent comprising alcohol with theprepolymer may be effected in a variety of ways under a varietyprocessing conditions such that the desired crystallinity of theprepolymer is obtained. The crystallinity in the prepolymer aftertreatment with the crystallization agent is preferably in the range ofbetween about 5 and 70% preferably between about 20 and 40%, even morepreferably between about 22 and 30% of the prepolymer.

Although a uniform coating of the crystallizing agent on the surface ofthe prepolymer is not required, it is desirable that contact is effectedso as to distribute the crystallizing agent on the surface ofessentially all the prepolymer. The crystallizing agent will preferablybe present on the prepolymer in an amount effective to effect thedesired crystallinity. The crystallization agent is preferably appliedto the prepolymer in amounts of from about 2 wt % to about 30 wt %, morepreferably in amounts of about 5 wt % to about 10 wt %, based on theweight of the prepolymer.

Contact of the crystallizing agent with the prepolymer may be effectedin any manner to achieve the desired crystallinity. As mentioned, thecrystallization agent may be in the vapor or liquid phase, or both.

If the crystallization agent is applied in liquid form, contact may beeffected by placing or submerging the prepolymer in a liquid bath of thecrystallizing agent, by spraying the crystallizing agent on theprepolymer, or by stirring, tumbling or agitating a mixture of theprepolymer and the crystallizing agent.

If the crystallization agent is in the vapor form, contact may beeffected by any means which brings the vapor in contact with theprepolymer. The prepolymer may be in any form suitable for processing,including, but not limited to, a pellet, a cellular pellet, anagglomerated pellet or a powder. For instance, contact may be effectedby placing the prepolymer in a vessel in which the crystallization agentis in the vapor form, or by placing the prepolymer in open orpressurized vessel in which the crystallization agent is in liquid formand subsequently heating the vessel to vaporize the crystallizationagent.

In one embodiment, in which contact of the crystallizing agent with theprepolymer is effected in the vapor phase, the prepolymer is introducedinto a pressurizable vessel and suspended on a screen. The crystallizingagent comprising an alcohol is introduced into the vessel and heated tothe boiling point of the alcohol for a period of time sufficient toproduce the desired crystallinity of the prepolymer. In an alternativeembodiment, the vapor may also be heated and subsequently introducedinto a pressurizable vessel.

In a further embodiment of the invention, pellets are placed in anevacuated vessel, which may be heated. The vessel is heated to atemperature such that the alcohol evaporates immediately upon theintroduction of the crystallization agent into the evacuated vessel. Itis preferable that the alcohol be completely in the vapor phase in thisembodiment.

It is preferable to effect contact of the crystallization agentcomprising the alcohol at a temperature of at least about 75° C., thetemperature defined by the relationship:

T_(c)≧T_(b) −z

wherein T_(c) is the contact temperature, T_(b) is the boiling point ofthe crystallization agent (both in degrees C) and z is a constant whosevalue is 60° C.

If applied in the vapor phase, contact of the crystallizing agentcomprising the alcohol is preferably effected at temperatures betweenthe boiling point of the crystallization agent and 150° C. Preferablecontact temperatures are in the range of between about 140° C. to about150° C., more preferably about 145° C. The pressure may be maintained atany pressure as long as the prepolymer is in contact with thecrystallization agent.

Since the T_(g) of the polycarbonate prepolymer is preferably in therange of about 100° C. to about 150° C., more preferably about 110° C.,it may be necessary to ramp the temperature from below the T_(g) of theprepolymer up to the preferred contact temperatures, if, for instance,the T_(g) of the prepolymer is below the preferable contacttemperatures. This allows the prepolymer to develop a level ofcrystallinity that will prevent agglomeration at the contacttemperatures. This rate of heating may vary depending on the particularpolycarbonate composition, suitable heating rates for this purpose aretypically in the range of about 3 degrees Celsius/minute.

The time required for contact with the crystallization agent comprisingthe alcohol will vary according to the composition of thecrystallization agent and the conditions of contact, such as thequantity of crystallization agent available and temperature. Generally,times in the range of about 15 to about 60 minutes are sufficient toachieve the desired level of crystallinity. The desired level ofcrystallinity is the level of crystallinty necessary to allow SSP totake place without fusion of the particulate prepolymer.

The choice of whether to use liquid or vapor phase contact of thecrystallization agent depends on a number of factors. One factor is thatthe prepolymer comprises low molecular weight oligomers, herein referredto as “lows”. Lows have a number average molecular weight of less thanabout 1500. If the crystallization agent is applied in the liquid phase,the lows tend to leach out of the prepolymer, thereby creating materialthat must be recycled or removed from the process. If thecrystallization agent is applied in the vapor phase, the lows tend toremain in the prepolymer. However, it is advantageous to leach the lowsout of the prepolymer as the molecular weight of the crystallizedprepolymer in subsequent SSP increases at a faster rate.

The polycarbonate prepolymer treated with the crystallization agentpreferably comprising at least about 95% by weight of an alcohol asdescribed herein are especially advantageous for use as a prepolymerfeedstock for solid state polymerization to produce higher molecularweight polycarbonates. Accordingly, the present process may beespecially advantageous as part of an overall process for making ahigher molecular weight polycarbonate, for example, by solid statepolymerization.

Addition of Additives to the Crystallization Agent

Optionally, the crystallization agent comprising the alcohol maycomprise an additive. An additive is a component in the crystallizationagent, in addition to the alcohol. Suitable additives include, but arenot limited to polymerization processing aids such as plasticizers, moldrelease agents, ketones or mixtures thereof. Other additives may includediluents or other mixing agents, including for example water and dialkylcarbonates.

Suitable plasticizers which may be included as additives in thecrystallization agent, include, but are not limited to, tetraethyleneglycol dimethyl ether, dioctyl phthalate, dibutyl phthalate, n-butylstearate, triethylene glycol di(caprylate-caprate) glycoeryl trioleate,di(2-ethylhexyl sebacate) and mixtures thereof. Typically, plasticicersreduce the T_(g) of the prepolymer.

Suitable mold release agents which may be included as additives in thecrystallization agent include, but are not limited to glycerolmonostearate, pentaerythritol tetrastearate and mixtures thereof.

Suitable ketones which may be included as additives in thecrystallization agent include, but are not limited to, methyl ethylketone, acetone, 4 methyl pentanone, cyclohexanone and isobutyl methylketone. Acetone is the more preferred ketone.

The additive or additives preferably have boiling points such that theyare in the vapor phase when the alcohol is in the vapor phase.Typically, the additive or additives, if present, comprise from about0.5 to about 5% by weight of the crystallization agent, and the alcoholfrom about 95 to about 99.5% by weight of the crystallization agent.

The particular amount of a given additive or additives is dependent onthe nature of the additive and its effect on the processing of thepolycarbonate. The plasticizers and mold release agents shouldpreferably be applied in amount of from about 50 ppm to about 3% byweight of the prepolymer, more preferably from about 300 ppm to about 1%of the prepolymer, and preferably comprise no more than about 2% byweight of the crystallization agent.

In contrast, if a ketone is used as an additive in the crystallizationagent comprising alcohol, the ketone may comprise a greater percentage,by weight, of the crystallization agent than, for instance, thanplasticizers or mold release agent without introducing deleteriouseffects in processing. A ketone may comprise for instance, about 20% byweight of the crystallization agent, more preferably about 10% by weightof the crystallization agent, even more preferably from about 1% toabout 5% of the crystallization agent. Alternatively, a carbonylcontaining compound, such as dimethyl carbonate, may be substituted forall or a portion of the ketone, and used in the same proportions.

The polycarbonate that has been treated and crystallized may be used inthe same plant, stored for later use, or packaged for transport, all incommercial quantities. Depending on the use of the crystallizedpolycarbonate and available equipment, the skilled worker may determinethe most desirable form for the crystallized prepolymer, i.e., pellet,powder or agglomerated pellet.

SSP of Polycarbonate Prepolymer Treated with Crystallization Agent

Optionally, the prepolymer that has been crystallized by effectingcontact with the crystallization agent may be converted to a polymer ofhigher molecular weight by solid state polymerization. The crystallizedprepolymer is maintained in the solid phase by maintaining theprocessing temperature below the melting temperature and above the glasstransition temperature of the prepolymer to yield a high molecularweight polymer. Several unexpected and advantageous effects werediscovered in polycarbonate prepolymer treated with a crystallizationagent preferably comprising at least about 95% by weight of an alcohol.

In a preferred embodiment, aromatic polycarbonate having a weightaverage average molecular weight in the range of about 5,000 to about200,000, preferably in the range of 10,000 to about 50,000, morepreferably in the range of about 15,000 to about 40,000 is formed.

The reaction temperature and time vary with the type and shape of thecrystallized prepolymer, the presence or absence of a catalyst orprocessing aid in the prepolymer, the level of crystallinity of theprepolymer, and the morphology of the prepolymer.

It is necessary, however, to maintain the processing temperature atabove the glass transition temperature of the crystallized prepolymerwhile maintaining the crystallized prepolymer in a solid phase statethroughout the solid state polymerization process. It is preferable toconduct the SSP reaction by maintaining the temperature of the reactionsystem less than but as close to the T_(m) as possible as follows:

T_(m)−10<T_(p)<T_(m)

where T_(m) is the melting temperature of the prepolymer in degreesCelcius, T_(p) is the reaction temperature in degrees Celcius.

As a non-limiting example, a suitable temperature for bisphenol Apolycarbonate is generally in the range of about 180° C. to about 260°C., preferably in the range of about 220° C. to about 245° C.

The processing temperature should be between the T_(g) and the T_(m) ofthe crystallized polycarbonate; for aromatic polycarbonates thistemperature is generally in the range of about 180° C. to about 260° C.,more preferably about 220° C. to about 245° C.

It was unexpectedly found that the use of a crystallization agentcomprising a secondary alcohol in the liquid or the vapor phase produceddesirable effects in subsequent solid state polymerization of theprepolymer. In particular, the use of a crystallization agent comprisinga secondary alcohol results in less thermal crystallization insubsequent SSP than the use of primary or tertiary alcohols.

Typically, a polycarbonate oligomer will thermally crystallize within afew minutes if held above about 200° C. The SSP reaction of thisoligomer therefore results in a competition between the rate ofpolymerization and the rate of thermal crystallization. If the rate ofthermal crystallization is faster, the polymerization rate is limited bythe high crystalline fraction that inhibits chain mobility and thediffusion of phenol from the solid particulates. It is highly desirable,therefore to control the rate of thermal crystallization. During solidstate polymerization, low molecular weight oligomers undergo thermalcrystallization in addition to the crystallization needed at the startof the process.

Polymerization and thermal crystallization are two competing processesduring SSP. Chain mobility is impeded by thermal crystallization and thedifference between ΔH initial and ΔH final represents the amount ofthermal crystallization during SSP. Uncontrolled crystallinity above ΔHfinal of about 45 joules/gram results in a precipitous fall in thereaction, and above about 60 joules/gram polymerization practicallystops. Therefore, it is important to keep thermal crystallization incheck.

Crystallization of the prepolymer with a secondary alcohol was found toretard the process of thermal crystallization in subsequent solid statepolymerization. In one embodiment, the agent comprising the alcohol isessentially pure secondary alcohol in the vapor phase. In order toachieve the desired effect of reduced thermal crystallization, at leastabout 3%, by weight of the prepolymer, of secondary alcohol should beapplied to the prepolymer.

In the solid state process described herein, staged heating may beemployed, with rapid ramping from one temperature level to the nextbetween instances of maintenance of the temperature at a specific level.Optionally, a large amount of inert gas, such as nitrogen, is used toensure rapid ramping from one temperature to the next and to impedethermal crystallization. Typically, a solid state polymerizationreaction must be conducted in at least four stages, as the ramping oftemperature is necessary to prevent agglomeration of the polycarbonate.

It was further unexpectedly found that polycarbonate prepolymer treatedwith a crystallization agent comprising an alcohol yields a prepolymerwhich may be processed at higher temperatures during solid statepolymerization, allowing the process to be conducted in fewer stages. Inparticular, polycarbonate prepolymer treated with a crystallizationagent comprising preferably at least about 95% by weight of an alcoholmay be polymerized at starting temperatures at least about 40° C. higherthan polycarbonate prepolymer treated by solvents, such as acetone. Thepolycarbonate prepolymer treated with the crystallization agentcomprising at least about 95% by weight of an alcohol may be processedin two stages.

In particular, the treated polycarbonate prepolymer may be treated in atwo stage process, where the first stage may be conducted at anoperating point of from about 210° C. to about 220° C., more preferablyabout 220° C. It is desirable to reach the first operating point as soonas possible in order to reduce the concomitant thermal crystallizationthat accompanies the high temperature. After a period of time at thistemperature, preferably from about 1 minute to about 2 hours, morepreferably about 1 hour, the temperature is ramped to from about 230° C.to about 240° C., more preferably about 240° C., as quickly as possible,and the reaction is completed at this temperature for a period of fromabout one to about four hours. It is preferable that the change to thesecond operating temperature be made immediately after the first stage.By “immediately” it is meant that the change be conducted as quickly aspossible, depending on the system capabilities, preferablyinstantaneously.

In addition to the advantages described above, it was furtherunexpectedly found that polycarbonate prepared from prepolymer treatedwith an agent comprising a secondary alcohol produced higher molecularweights in subsequent solid state polymerization than polycarbonateprepolymer treated with tertiary or primary alcohol; and thatpolycarbonate prepared from prepolymer treated with an agent comprisinga primary alcohol produced lower molecular weights than polycarbonateprepared from primary alcohol It was further found that among primaryalcohols, branched primary alcohols produced higher molecular weights.

The rate of thermal crystallization in subsequent solid statepolymerization of the crystallized prepolymer may also be controlled byaccelerating the rate of solid state polymerization. As the prepolymeris brought to temperatures at which the SSP reaction occurs, theprepolymer which has been crystallized by effecting contact with thecrystallization agent will undergo addition crystallization, whichimpedes the SSP reaction. Therefore, it is desirable to increase therate of SSP to overcome the effect of additional crystallization causedby the thermal crystallization.

Prior to introduction into the SSP reaction system, residual moisturemay be removed from the polycarbonate prepolymer by vacuum or a streamof heated gas. A stream of hot nitrogen is suitable for this purpose,for instance.

Suitable reaction systems for the solid state process include, but arenot limited to, fixed bed, moving bed, tumbler, batch, continuous or acombination thereof for any of the steps including preliminarypolymerization, crystallization and SSP.

During the SSP process, the process is accelerated by removing anaromatic monohydroxy compound or compounds and/or diaryl carbonateformed as by-products from the system. For this reason, an inert gas,such as nitrogen, argon, helium, carbon dioxide or a low molecularweight hydrocarbon gas may optionally be introduced to remove thearomatic monohydroxy compound or diaryl carbonate compound along withthe gas when it exits from the system and/or the reaction is conductedunder reduced pressure. When a gas is introduced into the reactionsystem, it is preferable to heat the gas to a temperature close to thereaction temperature before introducing it to the system.

Although the shape of the crystallized prepolymer used to execute theSSP reaction is not particularly restricted, it is noted that largemasses react slowly and are difficult to handle, and that pellets,beads, granules and powder are better suited shapes for SSP.

Acceptable forms for the prepolymer, include, but are not limited to,pellets, cellular pellets, as discussed in section II of thisspecification, beads, agglomerated pellets, powder and granules.Further, the crystallized prepolymer may optionally be sized suitablefor the solid state polymerization process by, for example, extrusionand die face cutting.

It was further found that polycarbonate produced by solid statepolymerization utilizing the process of this invention contained lessthan about 400 ppm, in particular less than about 300 ppm of Friesproduct in the embodiment in which the polycarbonate prepolymer isprepared by the melt process. The formation of the prepolymer by themelt process is preferred. Polycarbonate produced by the interfacialprocess typically has a Fries content of less than 5 ppm, and nearlyalways has a Fries content of below about 35 ppm. Polycarbonate producedby the melt process typically has higher Fries content. As used hereinthe term “Fries” or “fries” refers to a repeating unit in polycarbonatehaving the following formula (I):

Variable R_(c) and R_(d) each independently represent a hydrogen atom ora monovalent hydrocarbon group and may form a ring structure. VariableR_(e) is a divalent hydrocarbon group.

It is very desirable to have a low Fries content in the polycarbonateproduct, as Fries products reduce the performance characteristics of thepolycarbonate, such as the ductility. Higher Fries contents results inlower ductility. Preparing polycarbonate by the melt process results inthe formation of Fries products. In the embodiment where the prepolymeris prepared by the melt process, it was found that the SSP process didnot add additional Fries products to the product polycarbonate. Whilethe formation of the prepolymer by the melt process was found to produceFries products in the amount of about 200 to 300 ppm, the SSP processaccording to the invention did not produce any additional Fries product.

Effect of Additives in Crystallization Agent in Rate of Subsequent SSPof Polycarbonate

It was further unexpectedly found that certain additives to thecrystallization agent comprising alcohol produce an increase in the rateof subsequent solid state polymerization of the crystallizedpolycarbonate oligomer. These additives, effective to increase the rateof subsequent solid state polymerization of polycarbonate, include, butare not limited to, plasticizers, ketones and mold release agents.

Suitable plasticizers for increasing the rate of subsequent SSP,include, but are not limited to, tetraethylene glycol dimethyl ether,dioctyl phthalate, dibutyl phthalate, n-butyl stearate, triethyleneglycol di(caprylate-caprate) glycoeryl trioleate, di(2-ethylhexylsebacate) and mixtures.

Suitable mold release agents for increasing the rate of subsequent SSPinclude, but are not limited to glycerol monostearate, pentaerythritoltetrastearate and mixtures thereof.

Suitable ketones for increasing the rate of subsequent SSP, include, butare not limited to 4-methyl 2-pentanone, acetone, isobutyl methylketone, and cyclohexanone.

If the crystallization agent comprises primary or tertiary alcohols itis preferable to include an additive effective to increase the rate ofsolid state polymerization in the crystallization agent. Typically, theadditive or additives effective to increase the rate of subsequent SSP,if present, comprise from about 0.5 to about 5% by weight of thecrystallization agent, and the alcohol from about 95 to about 99.5% byweight of the crystallization agent.

If present in the crystallization agent as an additive effective toincrease the rate of solid state polymerization, the plasticizer and/ormold release agent which increases the rate of subsequent SSP shouldpreferably be included in the crystallization agent in amounts of fromabout 50 ppm to about 3% by weight of the prepolymer, more preferablyfrom about 300 ppm to about 1% by weight of the prepolymer. Theplasticizer or mold release agent should comprise no more than about 2%by weight of the crystallization agent.

In contrast, if a ketone is used as an additive effective to increasethe rate of solid state polymerization in the crystallization agentcomprising alcohol, the ketone may comprise a greater percentage, byweight, of the crystallization agent than, for instance, theplasticizers or mold release agent without introducing deleteriouseffects in processing. A ketone may comprise for instance, about 20% byweight of the crystallization agent, more preferably about 10% by weightof the crystallization agent, even more preferably from about 1% toabout 5% of the crystallization agent.

It was further found that other carbonyl containing compounds, such asdialkyl carbonates, function in the same manner as ketones in increasingthe rate of solid state polymerization, and may be used in the sameproportions as described in reference to the ketones. Suitable dialkylcarbonate for this purpose include, but are not limited to, dimethylcarbonate.

II. Polymer Pelletization

As mentioned, in a second aspect, the invention relates to a method ofpreparing cellular pellets from a prepolymer comprising a blowing agent,the method comprising:

a) extruding the prepolymer through a die having an output, the diemaintained at conditions such that the blowing agent remains in thecondensed phase in the prepolymer prior to emerging from the die; and

b) upon emergence of the prepolymer through the die, substantiallysimultaneously cooling the prepolymer by contacting the prepolymer witha cooling agent and cutting the prepolymer; the conditions at the outputof the die being maintained such that the blowing agent vaporizes in theprepolymer to form pores.

The invention further relates to cellular pellets prepared by thismethod, and articles prepared from the cellular pellets.

“Cooling agent” as used herein is defined as a medium which functions asa material to remove the heat of the treated prepolymer so that theprepolymer may be transformed from the molten state to the solid stateat a faster or more effective rate. The cooling agent may be any fluidor substance that can absorb or conduct heat. For example, the coolingagent may be pure water or a fluid comprising water in continuouscirculation to absorb heat through convection. Other cooling agents maybe utilized upon exit from the die. Other cooling agents include, butare not limited to crystallization agents comprising an alcohol and amixture of water and an agent which effects crystallization of thepolycarbonate prepolymer.

“Substantially simultaneously” as used in reference to the cutting andcooling steps means that although the cutting of the prepolymertypically occurs before the cooling of the prepolymer, the time intervalbetween the cutting and cooling is very short, typically a fraction of asecond.

Pelletization is an operation common to the manufacture of manysynthetic thermoplastic materials. Pelletization may be used to reducepolymers to a form suitable for storage and shipping.

The pellets formed according to the method of this invention areparticularly suited for use in solid state polymerization (SSP). Asdiscussed, partial crystallization of amorphous polycarbonate isrequired before SSP can be conducted. A partially crystalline shellinsures that the prepolymer can sustain a temperature higher than itsT_(g) without fusing with other prepolymer. In SSP, the prepolymer maybe in a variety of forms, including, but not limited to, powder,agglomerated pellets, and pellets.

The use of pellet forms is common in SSP as pellets are easily handledand prepared for processing. Pellets readily flow into measuring anddispensing apparatuses and the size of pellet charges can be readilycontrolled to small tolerances. Further, unlike powders, pellets areless prone to formation of dust and thus inadvertant exposure to them islimited. Thus, they provide a highly convenient form for the packaging,storage and use of thermoplastic polymers.

The method as described in this aspect of the invention may be appliedto all prepolymers. It is particularly useful when subsequentcrystallization and high surface area are required. Prepolymers arepolymers or oligomers which are intended as feedstock for a polymerhaving a higher molecular weight. The molecular weight of the prepolymermay vary depending on the particular polymer being processed.

Polycarbonate prepolymer for example, preferably have a weight averageaverage molecular weight in the range of from about 1,000 to about20,000; and may be processed further to produce polycarbonate havingweight average average molecular weights higher than about 20,000. Incontrast, polyester prepolymers typically have higher number averagemolecular weights. The term polymers as used herein includeshomopolymers, copolymers terpolymers and other combinations and forms ofpolymeric materials, including polymer which are elastomeric in nature.

Preferably, the prepolymer is a polymer which has properties suitablefor later SSP processing. Polymers suitable for later SSP processingwhich may be processed according to this aspect of the inventioninclude, but are not limited to polyesters, polyamides andpolycarbonates. Particular examples suitable for processing according tothis aspect of the invention include, but are not limited topolyhexymethylene adipamide (nylon 66), polyethylene terephthalate(PET), poly(butylene terephthalate) and bisphenol A polycarbonate.

The crystalline shell of pellets provides a region of higher porosityfavorable for SSP, however, the pellet surface area typically increasesfrom about 0.004 to about 0.37 m²/gram due to a network of cracksbrought about by local densification from the crystallization process.While suitable for obtaining polymer having weight average molecularweights in the range of about 15,000 to about 30,000, there still existsa need for polymer weight average molecular weights in the range ofabout 30,000 to about 80,000.

In one embodiment of the invention, the process of underwaterpelletization is utilized to prepare cellular pellets, which mayoptionally be further processed by SSP. Underwater pelletizers arecommonly used pelletization systems and a provided by a number ofmanufacturers such as GALA Industries of Eagle Rock, Va.

In an underwater pelletization system, molten polymer is extrudedthrough die orifices into a housing which is flooded with water providedfrom a water circulation system which is generally temperaturecontrolled. Cutting means, such as knives, rotating in close proximityto the die face, cut off the emerging strands of molten polymer to formpellets which are cooled in the water. The pellet and water slurry isthen removed from the housing for further processing steps such asdewatering and drying. The temperature of the water is preferably in therange of about 80° C. to about 100° C., more preferably about 90° C.

Although underwater pelletization is a preferred means for preparingcellular pellets according to this aspect of the invention, othersystems may be utilized to produce pellets having the desiredmorphology.

In the present invention, the prepolymer feedstock which is subjected tothe pelletization process comprises a blowing agent that has sufficientvapor pressure such that it vaporizes inside the heated melt afterexiting the die-head of the extruder. Upon exiting the die thevaporization of the blowing agent in the feedstock produces pores or“cells” in the prepolymer. Upon exiting the die-head, the hot melt isimmediately cut by a rotating cutter and cooled by a cooling agent, suchas water. This sudden cooling immediately solidifies the prepolymer andtraps the bubbles inside the pellets, thereby producing cellularpellets.

The cellular pellets produced by this method are particularly suitablefor use in SSP processes. When pellets of polycarbonate prepolymer weresubsequently crystallized, they yielded surface area and internal porestructure that resulted in faster reaction rates during the subsequentSSP of the prepolymer. The cracks and voids in the cellular pelletallows crystallinity to occur in the core of the pellet, as opposed toonly the shell thereby producing an internal structure to increasepellet integrity.

Cellular pellets prepared from polycarbonate prepolymer were determinedto have specific surface areas up to about 60% higher than pellets ofpolycarbonate prepolymer prepared by the process which yields pelletswithout cells, or “standard” pellets.

The cellular pellets produced according to the method of the inventionhave an increased internal surface area of up to about 0.7m ²/gram. Thisincreased surface area promotes faster reaction during solid statepolymerization. Without being bound by any theory, it is believed thatthe increased surface area allows enhanced diffusion paths for byproducts of SSP. For polycarbonates, for instance, the increased surfacearea would allow enhanced diffusion paths for phenol evolution.

In the embodiment in which cellular pellets are formed, the prepolymermelt must contain a “blowing agent”. The blowing agent may be aby-product of the formation of the prepolymer, the blowing agent may beintroduced into the prepolymer feedstock, or both.

In one embodiment, the prepolymer comprises an aromatic polycarbonateprepolymer having a weight average molecular weight of from about 1,000to about 20,000 and having from about 5 to about 95 mole % arylcarbonate terminal end groups. During the preparation of the prepolymerfeedstock, phenol and diphenylcarbonate are produced as by-products. Thephenol is suitable as a blowing agent.

In addition to using by-products present in the polycarbonate prepolymeras blowing agents, a variety of blowing agents may be added to the meltto create bubbles or voids. For example, introduction of easilycompressible gases, such as flourinated hydrocarbons, or of simple inertgases, such as nitrogen, into the melt during the pelletization processat wt % levels comparable to those described for phenol would effect thesame result.

The blowing agent, whether present as a by-product, or added to thepolymer feedstock, may be present in the prepolymer feedstock in varyingamounts. Suitable ranges include from about 100 ppm to about 5 weight %,based on the weight of the prepolymer, more preferably about 0.25 toabout 1% by weight, based on the weight of the prepolymer.

The blowing agent or volatile species should remain in the condensedphase or dissolved in the prepolymer at the conditions in the die headprior to discharge into the area of the second pressure. If the pressurein the die-head decreases below the vapor-pressure of the blowing agent,the vapors will escape the prepolymer melt before contacting the coolingagent. In order to obtain cellular pellets, it is important to trap asufficient portion of the vapor bubbles in the pellets to get thedesired cellular structure. In FIG. 1, the saturation vapor pressure ofpure phenol and pure diphenylcarbonate are plotted as a function oftemperature to illustrate a range of appropriate extrusion conditions.It is important to trap a sufficient portion of the vapor bubbles insidethe pellets to obtain the desired cellular structure. Similar diagramsfor other blowing agents in the prepolymer allow determination ofsuitable die conditions for the production of cellular pellets.

The size and internal morphology of the pellets is dependent on theextrusion conditions. In one embodiment, the phenol present in thepolycarbonate prepolymer functions as the blowing agent. In thisembodiment, the extrusion is preferably conducted at prepolymerfeedstock temperatures of from about 190° C. to about 225° C., morepreferably about 220° C. to about 225° C., even more preferably about223° C., and pressures of atmospheric to about 100 psi.

It is preferable to maintain a constant flow rate of the prepolymer inthe extruder and at conditions such that foaming of the blowing agentdoes not occur. If the conditions in the system are not maintainedproperly, hollow pellets or foamed pellets insufficient for handling mayresult. Hollow or foamed pellets are fragile due to low density.

In one embodiment, the prepolymer is fed into an extruder in the form ofa solid, such as a powder, cellular pellet, or agglomerated pellet. Theextruder is than ramped from a first temperature and atmosphericpressure to a first pressure in the die-head. Upon exiting the die-headthe melt enters a zone at a second pressure, at which the blowing agentvaporizes to produce the cellular pellets. Upon entering the zone ofsecond pressure, the prepolymer melt is substantially simultaneouslycooled and cut to produce the cellular pellets. Alternatively, theprepolymer may be directly fed to the pelletizer in the form of a melt.

The first and second pressures may be varied, however the secondpressure should be maintained lower than the first pressure. It isdesirable to maintain a pressure differential of at least about 100 psibetween the first and second pressures. In one embodiment, the secondpressure in maintained at ambient pressure and the first pressure ismaintained at about 100 psi. Alternatively, the second pressure may bemaintained at pressures other than ambient; however, the pressuredifferential between the first and second pressures should preferably beat least about 100 psi.

The size of the pellet formed is dependent on the processing conditions,including the flow rate of the prepolymer and the cutting speed. It isdesirable to obtain pellets of a uniform size, having a low aspectratio. If the pellet is too long, flowability may be impeded.

In general, it is desirable to obtain cellular pellets having agenerally spherical form. The length of the pellets is preferably in therange of about 2 mm to about 8 mm, more preferably about 2 mm to about 4mm, even more preferably about 3 mm. The width of the pellets ispreferably about 2 to about 3 mm, even more preferably about 2.5 mm. Theprepolymer melt flow rate and the cutting speed of the cutting means maybe synchronized to produce pellets of the desired dimensions.

In an alternative embodiment, a pellet of polycarbonate prepolymer maybe formed that does not have cells. These pellets are herein referred toas “standard” pellets. The standard pellets are produced in the samemanner as the cellular pellets, except that the extrusion is conductedat temperatures in the range of below about 190° C. These standardpellets, which do not contain the cells, are suitable for thepreparation of lower molecular weight materials, such as for use inoptical disks. Such materials have weight average molecular weights inthe range of from about 25,000 to about 35,000. The dimensions of thestandard pellets are preferably the same as those described for cellularpellets.

The standard pellets or cellular pellets may be used in the same plantafter being made, stored for later use, or packaged for transport, allin commercial quantities. If desired the standard pellets or cellularpellets may be crystallized in accordance with the method described insection I and used in the same plant after being made, stored for lateruse, or packaged for transport, all in commercial quantities.

SSP of Prepolymer Prepared by Pelletization Process

As mentioned, is desirable to use the cellular pellets in SSP processes.For polycarbonate for instance, the increased surface area translatesdirectly into faster removal of phenol, thereby increasing the rate ofSSP. Alternatively, the standard pellets may be used to preparematerials having lower molecular weight, such as for optical diskapplications.

In one embodiment, the cellular pellets from an underwater pelletizationprocess may be dewatered and introduced into a reactor in which SSP maybe effected. Prior to or after introduction into the reactor, thecellular pellets are crystallized to a level sufficient to allow SSP toproceed effectively.

If the prepolymer is a polycarbonate prepolymer, for instance, theprepolymer is subjected to a crystallization step. In a preferredembodiment, the cellular pellets prepared from the polycarbonateprepolymer, as described in section 1, is crystallized in accordancewith the method described in Section I of this specification.

Alternatively, other suitable nonsolvents or solvents may be used tocrystallize the polycarbonate prepolymer. Non-limiting examples ofsolvents useful in the present invention for crystallizing the amorphousprepolymer include aliphatic halogenated hydrocarbons, such as methylchloride, methylene chloride, chloroform, carbon tetrachloride,chloroethane, dichloroethane, trichloroethane, trichlorethylene,tetrachlorethane and mixtures thereof; aromatic halogenatedhydrocarbons, such as chlorobenzene and dichlorobenzene, ethers, such astetrahydrofuran and dioxane esters such as methyl acetate and ethylacetate, ketones such as acetone and methyl ethyl ketone; and aromatichydrocarbons such as benzene, toluene and xylene. Preferred solventsinclude ethers such as tetrahydrofuran and dioxane, esters such asmethyl acetate and ethyl acetate; and ketones such as acetone and methylethyl ketone. Acetone is the more preferred solvent. Optionally, watermay be added as a diluent to the solvent.

The SSP process may be conducted under a variety of processingconditions such that a polymer of the desired molecular weight isobtained. If the prepolymer is polycarbonate treated with thecrystallization agent described in section I of the specification, theprocess may be conducted in two stages with the advantages described inSection I, above.

This invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

EXAMPLES

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how thecompositions of matter and methods claimed herein are made andevaluated, and not intended to limit the scope of what the inventorsregard as their invention. Efforts have been made to insure accuracywith respect to numbers (e.g., amounts, temperatures, etc.) but someerror and deviations should be accounted for. Unless indicatedotherwise, parts are by weight, temperature is in °C. or is at roomtemperature. In the processes described, unless stated otherwise, thepressure is at or near atmospheric.

The materials and testing procedures used for the results shown hereinare as follows:

Molecular weights are reported as weight average (M_(w)), unless notedotherwise, and were determined by gel permeation chromatography relativeto polystyrene.

Number average molecular weights (M_(n)) were determined by gelpermeation chromatography.

T_(g) values were determined by differential scanning calorimetry.

T_(m) values were determined by differential scanning calorimetry.

In the following tables, the process steps may be read as follows:

220/1+230/4 would be interpreted as 1 hour at 220° C. followed by 4hours at 230° C.

Example 1

Samples of cellular pellets were produced using a GALA underwaterpelletizer. The temperature of the melt was 222.8° C. The pellets werecrystallized using alcohol vapor at a temperature of 150° C. with pureisopropanol vapor for one hour. The cellular pellets were produced atthe following conditions. The die and extruder temperature were 223° C.and the melt feed rate and cutting rate were speed synchronized toproduce pellets having a length of 3 millimeters (mm) and a diameter of3 mm. These pellets were designated as Gala #15 pellets. Gala #15pellets were cellular pellets. The cooling agent was water, and thetemperature of the water was 90° C. The die pressure was 100 psi.

The standard pellets were produced at the following conditions. Theconditions were the same as those set forth above for the Gala #15pellets, except that the extruder temperature was 190° C. The followingchart sets forth the properties of cellular pellets and standardpellets.

TABLE 1 Run Pellet Agent Temp(° C.)/Time/(hr) T_(g) T_(m) M_(w)*1000M_(n)*1000 M_(w)/M_(n) 1 Cellular MeOH 230/4 141 273 36 15 2.4 2Standard MeOH 230/4 139 263 25 11 2.3 3 Cellular MeOH 230/10 142 295 2812 2.3 4 Standard MeOH 230/10 133 284 21  9 2.3 5 Cellular 3- 210/1 +230/1 + 240/6 142 282 41 17 2.4 pentanol 6 Standard 3- 210/1 + 230/1 +240/6 137 270 29 11 2.3 pentanol

Example 2

This example shows that effecting crystallization with a crystallizationagent comprising an alcohol and a mold release agent as an additiveincreases the rate of solid state polymerization and provides moldreleasing properties in downstream processing.

A) 5.6 grams of 3-pentanol and 1% by weight of the polymer ofpentaerythritol tetrastearate (PETS) were charged into a pressurizablevessel and 12 grams of GALA #15 pellets were suspended on a screen. Thevessel was heated at a temperature of 150° C. for 1 hour, at thesaturation pressure of 3-pentanol of about 1.2 atmospheres, and thencooled to ambient temperature. The resultant crystalline pellets weresubjected to solid state polymerization according to the protocol givenin Table 2.

TABLE 2 Temp/Time T_(g) T_(m) Stage (° C./hr) (° C.) (° C.) M_(w) M_(n)1 Initial sample 116 212 15400  5400 2 200/0.5 + 220/0.5 + 230/1 148 25449150 21450 3 200/0.5 + 220/0.5 + 230/1 + 149.5 256 60400 26100 240/1 4200/0.5 + 220/0.5 + 230/1 + 151 256 67900 27100 240/1.5

B) 5.6 grams of 3-pentanol and 1% by weight of the prepolymer ofglycerol monostearate (GMS) were charged into a pressurizable vessel. 12grams of amorphous pellets were suspended on a screen. The vessel washeated at 150° C. for one hour, at the saturation pressure of 3-pentanolof about 1.2 atmospheres, and then cooled to ambient temperature. Theresultant crystalline pellets were subjected to solid statepolymerization according to the protocol given in Table 3.

TABLE 3 Temp/Time T_(g) T_(m) Stage (° C./hr) (° C.) (° C.) M_(w) M_(n)1 Initial sample 120 224 19600  6000 2 200/0.5 + 22010.5 + 230/1 149 25549100 21300 3 200/0.5 + 220/0.5 + 230/1 + 149 256 59500 24400 240/1 4200/0.5 + 220/0.5 + 230/1 + 152 260 64700 26500 240/1.5

Comparative Example C

C) 8 grams of pure 3-pentanol were charged in a pressurizable vessel and12 grams of amorphous pellets were suspended on a screen. The vessel washeated at 150° C. for 1 hour, at the saturation pressure of 3-pentanolof about 1.2 atmospheres, and then cooled to room temperature. Theresultant crystalline pellets were subjected to solid statepolymerization according to the protocol given in Table 4.

TABLE 4 Temp/Time T_(g) T_(m) Stage (° C./hr) (° C.) (° C.) M_(w) M_(n)1 Initial sample 111 222 2 200/0.5 + 220/0.5 + 230/1 143 256 40800 169003 200/0.5 + 220/0.5 + 230/1 + 147 259 47600 19600 240/1 4 200/0.5 +220/0.5 + 230/1 + 149 263 55000 23700 240/3

Example 3

This example shows that the alcohol, i.e., primary, secondary ortertiary, in the crystallization agent has an effect on the finalmolecular weight of the polymer.

The prepolymer used for this study was in the pellet form, approximately3 mm×9 mm. These pellets were exposed to different alcohol vapors at150° C., at the saturation pressure of the crystallization agent, for 1hour for crystallization. Primary, secondary and tertiary alcohols wereused for this study. The crystalline pellets were subjected to SSP inthe temperature range of 200 to 240° C. under a flow of nitrogen at atm.In all cases, crystallization and SPP were performed under identicalconditions.

Alcohol (10 gm) was charged in a crystallization apparatus consisting ofa pressurizable vessel and amorphous pellets (11.5 g) were suspended onthe screen. The vessel was heated at 150° C. for 1 hour, at thesaturation pressure of the crystallization agent, and then cooled it toroom temperature. The resultant crystalline pellets were subjected tosolid state polymerization according to the protocol given in Table 1.

Alcohol Used for this Study:

Primary alcohol: methanol, isobutanol, 1-pentanol

Secondary alcohol: 2-propanol, 3-pentanol

Tertiary alcohol: t-butanol

From the results, shown in Table 5, it was found that PC crystallizedfrom secondary alcohol produce highest molecular weight whereas primaryalcohols produce lowest molecular weight. Again among the primaryalcohol when there is a branching, better results were obtained comparedto linear primary alcohol. The experimental data and results are givenbelow:

TABLE 5 Methanol 3-Pentanol 2-Propanol t-Butanol Isobutanol 1-PentanolTemp/time Tg Tg Tg Tg Tg Tg (° C./h) (° C.) Mw (° C.) Mw (° C.) Mw (°C.) Mw (° C.) Mw (° C.) Mw Initial Sample 109  9540 109  9540 109  9540109  9540 109  9540 109  9540 After crystallization 105  6530 114 11200121 16200 114 17270 114 13713  9995 200/0.5 + 230/0.5 + 128 18473 14847700 148 53400 146 42400 145 42882 135 27550 230/1 200/0.5 + 230/0.5 +133 23448 149 59870 151 63700 148 48450 148 44050 139 32570 230/1 +240/1 200/0.5 + 230/0.5 + 137 25508 151 64576 151 69950 149 55950 14848500 139 32350 230/1 + 240/2

Example 4

This example shows that controlling the crystallinity of the prepolymeris desirable in order to ensure efficient SSP, and that secondaryalcohols help to minimize the process of thermal crystallization.Different types of alcohols as well as additives in alcohols can be usedto control thermal crystallinity during SSP, as given by ΔH final inTable 6. In particular, the addition of rate enhancing additives incrystallization agents comprising primary and tertiary alcohols helps tominimize the effects of thermal crystallization.

In this series of examples, 10 grams of the alcohol and 1% of theadditives by weight of the prepolymer were charged into a pressurizablevessel with 11.5 grams of amorphous pellets suspended in a screenbasket. Gala #15 pellets were contacted with vapor phase alcohol. Thevessel was heated at 150° C. for 1 hour and at the saturation pressureof the alcohol, and then cooled to ambient temperature. The resultantcrystalline pellets were subjected to solid state polymerizationaccording to the protocol shown in Table 6.

TABLE 6 Reaction T_(m) T_(m) ΔH ΔH Temp/time Initial Final initial finalRun System (° C./h) ° C. ° C. J/gm J/gm M_(w) M_(n) 1 Methanol 200/0.5 +225 291 39 68 25500  9408 220/0.5 + 230/1 + 240/2 2 Hexanol 220/3 +230/2 + 224 299 22 61 48000 18800 240/8 3 3-pentanol 220/3 + 230/2 + 222284 25 44 56000 23100 240/8 4 3-pentanol 200/0.5 + 220/ 263 31 5500023750 0.5 + 230/1 + 240/3 5 Methanol + 200/0.5 + 220/ 225 280 34 6231750 11700 TEGDME 0.5 + 230/1 + (1%) 240/1.5 6 3-pentanol + 200/0.5 +220/ 224 255 28 18 69700 24700 TEGDME 0.5 + 230/1 + (1%) 240/2 73-pentanol + 200/0.5 + 220/ 212 256 32 14 67900 27100 PETS (1%) 0.5 +230/1 + 240/2 8 3-pentanol + 200/0.5 + 220/ 224 260 25 22 64700 27300GMS (1%) 0.5 + 230/1 + 240/2

Example 5

This example shows that application of the crystallization agentcomprising a plasticizer increases the rate of solid statepolymerization of subsequently polymerized prepolymer.

A) 32 grams of 3-pentanol and 1% by weight with respect to the3-pentanol of glycol dimethyl ether(TEGDME) were charged into apressurizable vessel and 12 grams of GALA #15 pellets were placed on ascreen. The vessel was heated at 150° C. for one hour at the saturationpressure of the alcohol (approximately 1.2 atmospheres) and then cooledto room temperature. The resulting crystalline pellets were subjected tosolid state polymerization according to the protocol set forth in Table7.

TABLE 7 Temp/Time Run (° C./h) T_(g) T_(m) M_(w) M_(n) 1 Initial 2200/0.5 + 220/0.5 + 147 248 51300 19650 230/1 3 200/0.5 + 220/0.5 + 152260 66000 24100 230/1 + 240/1 4 200/0.5 + 220/0.5 + 69700 24700 230/1 +240/1.5

B) 8 grams of 3-pentanol and 1500 ppm, with respect to the 3-pentanol,of tetraethylene glycol dimethyl ether (TEGDME) were charged into acrystallization apparatus consisting of a pressurizable vessel and 12grams of GALA #15 pellets were placed on the screen. The vessel washeated at 150° C. for 1 hour at the saturation pressure of the alcoholand cooled to room temperature. The resultant pellets were subjected toSSP according to the protocol given in Table 8

TABLE 8 Temp/Time Run (° C./h) T_(g) T_(m) M_(w) M_(n) 1 Initial 122 2192 200/0.5 + 220/0.5 + 148 255 45400 20800 230/1 3 200/0.5 + 220/0.5 +150 258 54000 23800 230/1 + 240/1 4 200/0.5 + 220/0.5 + 150 257 6130028500 230/1 + 240/2

C) 8 grams of 3-pentanol and 1000 ppm, with respect to the 3-pentanol,of tetraethylene glycol dimethyl ether (TEGDME) were charged into acrystallization apparatus consisting of a pressurizable vessel and 12grams of GALA #15 pellets were placed on the screen. The vessel washeated at 150° C. for 1 hour at the saturation pressure of the alcoholand cooled to room temperature. The resultant pellets were subjected toSSP according to the protocol given in Table 9.

TABLE 9 Temp/Time Run (° C./h) T_(g) T_(m) M_(w) M_(n) 1 Initial 2200/0.5 + 220/0.5 + 146 258 45760 20900 230/1 3 200/0.5 + 220/0.5 + 149260 53450 24670 230/1 + 240/1 4 200/0.5 + 220/0.5 + 150 262 57280 26230230/1 + 240/2

Comparative Example D) (no additives)

D) 8 grams of 3-pentanol were charged into a crystallization apparatusconsisting of a pressurizable vessel and 12 grams of GALA #15 pelletswere placed on the screen. The vessel was heated at 150° C. for 1 hourat the saturation pressure of the alcohol and cooled to roomtemperature. The resultant pellets were subjected to SSP according tothe protocol given in Table 10.

TABLE 10 Temp/Time Run (° C./h) T_(g) T_(m) M_(w) M_(n) 1 Initial 111222 2 200/0.5 + 220/0.5 + 143 256 40800 16900 230/1 3 200/0.5 +220/0.5 + 147 259 47600 19600 230/1 + 240/1 4 200/0.5 + 220/0.5 + 149263 55000 23700 230/1 + 240/3

Example 6

This example shows that application of the crystallization agentcomprising a ketone increases the rate of solid state polymerization ofsubsequently polymerized prepolymer.

A) 8 grams of 3-pentanol were charged into a crystallization apparatusconsisting of a pressurizable vessel and 12 grams of GALA #15 pelletswere placed on the screen. In this instance, 2% of 4 methyl 2-pentanonewas added to 98% of 3-pentanol. The vessel was heated at 150° C. for 1hour at the saturation pressure of the alcohol (approximately 1.2atmospheres), and cooled to room temperature. The resultantellets weresubjected to SSP according to the protocol given in Table 11.

TABLE 11 Temp/Time Run (° C./h) T_(g) T_(m) M_(w) M_(n) M_(w)/M_(n) 1Initial 116 224 2 200/0.5 + 220/0.5 + 147 276 48000 21100 2.27 230/1 3200/0.5 + 220/0.5 + 149 269 60000 24350 2.46 230/1 + 240/2 4 200/0.5 +220/0.5 + 150 282 61000 26000 2.35 230/1 + 240/3 5 200/0.5 + 220/0.5 +153 273 65600 24500 2.57 230/1 + 240/4 6 200/0.5 + 220/0.5 + 155 26885000 32600 2.6 230/1 + 240/6

B) Comparative Example

In this example, no ketone was added. 8 grams of 3-pentanol were chargedinto a crystallization apparatus consisting of a pressurizable vesseland 12 grams of GALA #15 pellets were placed on the screen. The vesselwas heated at 150° C. for 1 hour and at the saturation pressure of the3-pentanol (approximately 1.2 atmospheres) and cooled to roomtemperature. The resultant pellets were subjected to SSP according tothe protocol given in Table 12.

TABLE 12 Temp/Time Run (° C./h) T_(g) T_(m) M_(w) M_(n) M_(w)/M_(n) 1Initial 111 222 2 200/0.5 + 220/0.5 + 143 256 40800 16900 2.41 230/1 3200/0.5 + 220/0.5 + 147 259 47600 19600 2.42 230/1 + 240/1 4 200/0.5 +220/0.5 + 149 263 55000 23700 2.32 230/1 + 240/3 5 220/3 + 230/2 + 2 154284 56000 23700 2.32 40/6

C) In this further example, according to the invention, 50 grams of GALA#15 pellets were exposed to a crystallization agent comprising isopropylalcohol (IPA) as set forth in Table 1. In experiment Number 1, thecrystallization agent comprised 5% acetone, by weight of thecrystallization agent. Crystallization was effected for 1 hour at 150°C. at the saturation pressure of the alcohol. The cellular pellets werepolymerized at 220° C. for 1 hour, followed by polymerization at 240° C.for 4 hours. During the process, an inert gas sweep of 3 liters/minuteof nitrogen was maintained.

TABLE 13 Solvent T_(g) T_(m) T_(m) onset ΔH Experiment No. (%) Mwt (°C.) (° C.) (° C.) (J/g) 1 IPA + 14550 106 228 185 30^(b) startingoligomer Acetone (95:5) Product 48500 147 277 240 44 2 IPA 12600 103 232181 29^(b) starting oligomer (100) Product 42680 144 275 260 53^(b)Starting oligomer

This invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. A method of preparing cellular pellets suitablefor use in solid state polymerization from a prepolymer comprising ablowing agent, the method comprising the steps of: a) extruding theprepolymer through a die, the die maintained at conditions such that theblowing agent remains in the condensed phase in the prepolymer prior toemerging from the die; b) upon emergence of the prepolymer through thedie, substantially simultaneously cooling the prepolymer by contactingthe prepolymer with a cooling agent and cutting the prepolymer; theconditions outside the die being maintained such that the blowing agentvaporizes in the prepolymer to form pores; and c) effecting contact ofthe cellular pellets with a crystallizing agent comprising at least 95%by weight of an alcohol.
 2. The method of claim 1, wherein theprepolymer comprises a polyester, a polyamide or a polycarbonate.
 3. Amethod of preparing cellular pellets suitable for use in solid statepolymerization comprising an aromatic polycarbonate prepolymer having aweight average molecular weight of from about 1,000 to about 20,000 andhaving from about 5 to about 95 mole % aryl carbonate terminal endgroups, based on total end groups, the method comprising: a) extrudingthe polycarbonate prepolymer through a die having an output, the diemaintained at conditions such that the blowing agent remains in thecondensed phase prior to emerging from the die; b) upon emergence of thepolycarbonate prepolymer through the die, substantially simultaneouslycooling the polycarbonate prepolymer by contacting the polycarbonateprepolymer with a cooling agent and cutting the polycarbonateprepolymer; the conditions at the output of the die being maintainedsuch that the blowing agent vaporizes in the prepolymer to form pores;and c) effecting contact of the cellular pellets with a crystallizingagent comprising at least 95% by weight of an alcohol.
 4. The method ofclaim 3, wherein the blowing agent is phenol.
 5. The method of claim 3,wherein the cooling agent is water.
 6. A method of treating apolycarbonate feedstock, the polycarbonate feedstock comprising anaromatic polycarbonate prepolymer having a weight average molecularweight of from about 1,000 to about 20,000 and having from about 5 toabout 95 mole % aryl carbonate terminal end groups, based on total endgroups, the feedstock further comprising a blowing agent, the methodcomprising: a) conveying the feedstock through an extruder, the extruderhaving a die maintained at a first pressure, the die having an output,the output maintained at a second pressure lower than the first pressureand at conditions such that the blowing agent vaporizes in thefeedstock; and b) effecting contact of the feedstock with acrystallizing agent comprising at least 95% by weight of an alcohol. 7.The method of claim 6, wherein the extruder is maintained at conditionssuch that the blowing agent remains in the condensed phase in theextruder.
 8. A method of preparing cellular pellets suitable for use insolid state polymerization, the cellular pellets comprising an aromaticpolycarbonate prepolymer having a weight average molecular weight offrom about 1,000 to about 20,000 and having from about 5 to about 95mole % aryl carbonate terminal end groups, based on total end groups,the method comprising: a) feeding a feedstock comprising thepolycarbonate prepolymer and a blowing agent to an extruder having adie, the die having an output, the extruder operating at a firstpressure; b) conveying the feedstock through the extruder to the output,the output maintained at a second pressure lower than the first pressureand at conditions such that the blowing agent vaporizes in thefeedstock; c) substantially simultaneously cutting and cooling thefeedstock at the output, thereby producing cellular pellets; and d)effecting contact of the cellular pellets with a crystallizing agentcomprising at least 95% by weight of an alcohol.
 9. The method of claim8, wherein the cooling is effected by water.
 10. The method of claim 8,wherein the blowing agent is phenol.
 11. The method of claim 8, whereinthe second pressure is ambient pressure.
 12. A method of preparingcellular pellets suitable for use in solid state polymerization, thecellular pellets comprising an aromatic polycarbonate prepolymer havinga weight average molecular weight of from about 1,000 to about 20,000and having from about 5 to about 95 mole % aryl carbonate terminal endgroups, based on total end groups, the method comprising: a) conveying afeedstock comprising the prepolymer through an extruder, the extrudercomprising a die operating at a first pressure, the die having an outputmaintained at a second pressure lower than the first pressure; b)introducing a blowing agent into the prepolymer prior to the emergenceof the prepolymer at the die; wherein the conditions at the output ofthe die are maintained such that the blowing agent vaporizes in theprepolymer at the output; c) substantially simultaneously cutting thefeedstock at the output and contacting the feedstock with water, therebyproducing cellular pellets and d) effecting contact of the cellularpellets with a crystallizing agent comprising at least 95% by weight ofan alcohol.
 13. A method of preparing cellular pellets suitable for usein solid state polymerization, the cellular pellets comprising anaromatic polycarbonate prepolymer having a weight average molecularweight of from about 1,000 to about 20,000 and having from about 5 toabout 95 mole % aryl carbonate terminal end groups, based on total endgroups, the method comprising: a) conveying a feedstock comprising theprepolymer and phenol through an extruder operating at a first pressureand having an output maintained at a second pressure lower than thefirst pressure, the output further maintained at conditions such thatthe phenol vaporizes in the feedstock; b) substantially simultaneouslycutting the feedstock at the output and contacting the feedstock withwater, thereby producing cellular pellets; and c) effecting contact ofthe cellular pellets with a crystallizing agent comprising at least 95%by weight of an alcohol.
 14. The method of claim 13, wherein the secondpressure is at or near ambient pressure.
 15. The method of claim 13,wherein cooling is effected by contact with water.
 16. The cellularpellets prepared by the method of claim
 1. 17. The cellular pelletsprepared by the method of claim
 3. 18. The cellular pellets prepared bythe method of claim
 8. 19. The cellular pellets prepared by the methodof claim
 12. 20. The cellular pellets prepared by the method of claim13.
 21. A method of preparing an aromatic polycarbonate having a weightaverage molecular weight of from about 30,000 to about 80,000 from anaromatic polycarbonate prepolymer having a weight average molecularweight of from about 1,000 to about 20,000 and having from about 5 toabout 95 mole % aryl carbonate terminal end groups, based on total endgroups, the aromatic polycarbonate prepolymer further comprising ablowing agent, the method of preparing the polycarbonate comprising: a)extruding the polycarbonate prepolymer comprising a blowing agentthrough a die having an output, the die maintained at conditions suchthat the blowing agent remains in the condensed phase prior to emergingfrom the die, and upon emergence of the polycarbonate prepolymer throughthe die, substantially simultaneously cooling the polycarbonateprepolymer by contacting the polycarbonate prepolymer with a coolingagent and cutting the polycarbonate prepolymer; the conditions at theoutput of the die being maintained such that the blowing agent vaporizesin the prepolymer to form pores, thereby forming a cellular pellet; b)effecting contact of the cellular pellets with a crystallizing agentcomprising at least about 95% by weight of an alcohol; and c) heatingthe crystallized cellular pellets to a reaction temperature above theglass transition temperature of the crystallized cellular pellets andbelow the melting temperature of the crystallized cellular pellets,thereby forming the aromatic polycarbonate.
 22. The method of claim 21,wherein the alcohol in the crystallizing agent is isopropanol.
 23. Themethod of claim 21, wherein the crystallization agent consistsessentially of isopropanol.
 24. The method of claim 21, wherein thecooling agent is water.
 25. The method of claim 21, wherein step d)consists of a first stage and a second stage, the first stage comprisingheating the cellular pellets at a temperature of from about 210° C. toabout 220° C. for a period of about one hour, the second stagecomprising heating the cellular pellets at a temperature of from about230° C. to about 240° C. for a period of from about one to about fourhours, wherein following the first stage, the temperature is ramped tothe second stage temperature immediately.